Recently, an electrographic image forming apparatus such as a laser printer, digital copy machine or laser facsimile is provided with a post-processing device (finisher) that staples a sheet bundle. The conventional finisher discharges a sheet bundle by using a roller and a bundle hook.
However, if the finisher discharges a sheet bundle by using the bundle hook, the rear end of the sheet bundle may not properly fall into a paper discharge tray because of the influence of the coefficient of friction of the sheets, of elasticity (strength) of the sheet bundle, and of the quantity of electricity with which the sheet bundle is charged. The insufficiently falling sheet bundle may be hooked by the bundle hook. This causes a problem of sheet jam.